I have an 87 Dodge Dakota V6, originally equipped with a Holley 6280
electronic feedback carburetor. The mixture control solenoid in the carb
went south a while back and is no longer available so I replaced the
6280 with a 2280, which is the non-feedback version without the mixture
control solenoid.

Recently, after installing the new carb and an MSD spark booster, and
adjusting everything to factory specs, I made a 200 mile trip which was
about 90% freeway and with a 1/2 mile gain in elevation.

I got 22.7 mpg. And this is with a heavy duty lumber rack and a 2'x2'x4'
toolbox full of tools and starting out with a full tank of gas. The
truck had about 113,000 miles on it.

I was really dumbstruck at the gas mileage I got on this trip and then
started to wonder how I could better it. The factory manual says
disconnect all the ignition timing inputs to the computer to set the
spark to 7 degrees BTC. That's where it is now.

Which way should I tweak the timing to get better mileage? Should I go 5
degrees BTC or 9 degrees BTC? I am especially interested in getting
better city mileage with lot's of stop and go driving. Can I expect this
 by tweaking the ignition timing?